Electron optical system

ABSTRACT

An electrostatic focus system, particularly suitable for electron optical image tubes includes a photocathode which provides a source of electrons corresponding to an input image. The electrons are focused to a point by a dual focus system including a pair of focusing electrodes. The first focusing electrode has an aperture through which the electrons are accelerated. The second electrode has a relatively smaller aperture and has a potential applied to it which causes deceleration of the electrons and sufficient convergence of the electrons to enable them to pass through the aperture. The electrons tend to focus to a point near the aperture of the second electrode due to the geometry of and the potentials applied to the electrode configuration. The electrons spread from the point of focus to form an image on an electron receiving surface, such as a phosphor coated target. The dual focus electrode system forms a lens which mainly compensates for geometric and chromatic aberration electron image distortion caused by a lens existing between the photocathode and the first focus electrode.

limited States Fatent [191 Each, Jr. et a1.

l l ELECTRON OPTICAL SYSTEM [75] Inventors: Richard S. Enck, Jr., Mountain View; James P. Sackinger, San Jose,

both of Calif.

i731 Assignee: Varian Associates, Palo Alto, Calif.

{22] Filed: June 28, 1973 [21] Appl. No.: 374,440

Primary Examiner-Richard A. Farley -isslstam Examiner-J. M. Potenza Attorney, Agent, or Firm-Stanley Z. Cole; D. R. Pressman; R. K. Stoddard [451 July 22,1975

[5 7] ABSTRACT An electrostatic focus system, particularly suitable for electron optical image tubes includes a photocathode which provides a source of electrons corresponding to an input image. The electrons are focused to a point by a dual focus system including a pair of focusing electrodes. The first focusing electrode has an aperture through which the electrons are accelerated. The second electrode has a relatively smaller aperture and has a potential applied to it which causes deceleration of the electrons and sufficient convergence of the electrons to enable them to pass through the aperture. The electrons tend to focus to a point near the aperture of the second electrode due to the geometry of and the potentials applied to the electrode configuration. The electrons spread from the point of focus to form an image on an electron receiving surface, such as a phosphor coated target. The dual focus electrode system forms a lens which mainly compensates for geometric and chromatic aberration electron image distortion caused by a lens existing between the photocathode and the first focus electrode.

11 Claims, 2 Drawing Figures PATENTEDJUL 22 ms 5 8 9633 1 FIG.2 7

ELECTRON OPTICAL SYSTEM FIELD OF THE INVENTION The present invention is related to electron optics and, more particularly, to a system and a method wherein a charged particle beam image is passed through accelerating and decelerating focusing fields prior to impinging on a target.

BACKGROUND OF THE INVENTION Many types of image distortions are known in electron optic devices which are used for controlling an electron beam image, such as is derived from a photocathode. One distortion, typically referred to as geometric distortion, manifests itself as either a barrel-or a pin-cushion-shaped output image derived from a rectangular input image. Geometric distortion is due to deviations of optical and electron input and output surfaces from ideally spherical surfaces. Thus, if a planar or spherical output surface is used, it may cut a different contour where the electron image is actually in focus to produce either a barrel or a pin-cushion shape. Spherical input and output surfaces are quite difficult to manufacture and present problems in coupling an electron image to an optical image or coupling an optical image to an electron image. One solution in the prior art is to use a fiber-optic plate to couple an optical image existing on one contoured surface to an electron image existing on another contoured surface. Generally, such coupling is provided between a planar optical surface and a spherical electron surface. This technique is somewhat effective but still may not eliminate some geometric distortion. Particularly, in image tubes where a large change in size is desired between an input image and an output image, distortions are apparent. As an example, if a large minification, i.e., reduction in size is desired, in an image tube, the electron emitting surface must be large, and there is difficulty in providing the proper focusing and accelerating fields in a reasonable length to produce a reduced image at the electron receiving surface.

The ability to focus electrons from a surface source corresponding to an input image depends in large part upon the strength of the voltage gradient which can be established normal to the electron emitting surface. The stronger the gradient, the more readily barrel or pin cushion distortions can be overcome.

A second type of distortion, termed chromatic aberration, results from a statistical distribution of the angle and the velocities at which electrons are ejected from an electron emitting surface, such as a photocathode. Due to the velocity distribution, electrons emitted from the same point on the photocathode follow a band of statistically described trajectories. Chromatic aberration is also minimized as the voltage gradient at the electron emitting surface is increased because the resultant band of trajectories is narrower. Chromatic aberration is particularly acute where a large electronemitting surface is used, as would be the case where a large minification is desired.

Thus, the provision of large voltage gradients at the electron-emitting surfaces is a key to the production of distortion-free images. However, in prior art devices, limitations have been placed on these initial voltage gradients by the size of the input and output surfaces and the overall distances between these surfaces. These limitations exist because a single aperture focus electrode has generally been used. In some instances, distortion correctors maintained at an accelerating potential have been used downstream from the first focus electrode. These correctors have not been able to correct for large amounts of distortion, particularly that distortion due to chromatic aberration as would be present in a minifying image tube. Also, mesh screens have been used in order to establish critical equipotential surfaces along the electron stream. These screens, however, have also not been entirely satisfactory because they are easily burned and because they intercept electrons to thereby decrease the resolution capabilities of the image tube.

SUMMARY OF THE INVENTION We have found that it is impossible to substantially reduce geometric and chromatic aberration distortions by passing a charged particle beam image (generally an electron beam) through accelerating and decelerating focus fields prior to the beam impinging on a target. A first focus electrode accelerates and converges the electrons emitted from the electron-emittingsurface. This first electrode may operate at a higher voltage, without causing image distortion, than generally possible with prior art devices to therefore produce large gradients at the electron emitting surface. The higher voltage can be achieved because a second, decelerating focus electrode is provided downstream or in back of the first electrode. The deceleration is provided by maintaining the second focus electrode at a lower potential than the first focus electrode. Once the electrons have been somewhat converged by the first focus electrode, they are then further converged by the second focus electrode. The space between the first and second focus electrodes may be viewed as a lens which compensates for pin cushion or barrel type geometric distortion, as well as compensating for chromatic aberration type distortion. The lens provides an exit stream of electrons which is mostly free of the distortion caused by a field existing between the electron emitting surface and the accelerating first focus lens. The distortion-free electron stream may be conveniently passed through means for zooming its size without disturbing the fields upstream of the region where zooming occurs. The dual-focus electrodes of the invention are also useful in conjunction with a generally spherical electron emitting surface or with a spherical electron receiving surface, as would be used where either a large minification or magnification of the input image is desired.

OBJECTS OF THE INVENTION It is an object of the present invention to provide an improved charged particle focusing system wherein barrel or pin cushion distortion is substantially eliminated.

A further object is to provide an improved charged particle focusing system having reduced chromatic aberration.

It is another object of the present invention to provide an improved electron optic system using planar input and output surfaces having improved focal plane flatness.

It is another object of the present invention to provide an auxiliary lens to compensate for geometric and chromatic aberration distortion caused by a lens effectively existing between an electron emitting surface and the usual focus electrode.

It is another object of the present invention to provide a distortion compensated electron image which may be electronically zoomed by a means downstream therefrom without interfering with an accelerating field in the vicinity of the electron source.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-sectional view of an image intensifier tube utilizing the electrostatic focusing system of the invention and having planar input and output surfaces with nominally unity magnification.

FIG. 2 is an alternate embodiment showing in cross section an image intensifier of substantial minification, having a large, generally spherical, input surface and a smaller, planar output surface.

DETAILED DESCRIPTION OF THE INVENTION Referring first to FIG. 1, there is shown in an evacuated envelope 3 a substantially unity magnification image intensifier tube having a planar input charged particle emitting surface 2. Surface 2 is a photocathode for forming an electron beam image corresponding to an input light image 4. By conventional optics, an optical image 6, corresponding to the input light image, is focused on one side of the photocathode 2 by lens 8. Optical rays and 12 are exemplary of rays within an optical bundle of rays impinging on photocathode 2. These rays are respectfully transduced by the photocathode into electron trajectories l6 and 18 that form outer boundaries of an electron beam. This beam propagates within the tube envelope 3 past various electron optic parts and finally impinges on a target electrode 36 where an output electron image is formed which can be optically read or converted to an electrical signal. Since the electrons emitted from any given point of the electron emitting surface 2 follow a band of trajectories, due to chromatic aberration, each of the trajectories l6 and 18 is actually representative of the average path of the electrons in a band of trajectories.

The electron beam emitted from photocathode 2 is focused onto target 36 with minimal chromatic aberration and geometric distortion by including a focusing structure comprising frusto-conically shaped coaxial accelerating and decelerating electrodes and 28 having aligned apertures 22 and 30, respectively, through which the electron beam propagates. Electrodes 20 and 28 are symmetrical about the electron beam axis and are formed by revolving a line about the electron beam axis which is inclined inwardly and upstream. The inclined walls of the electrodes 20 and 28 are sloped so that the distance between them and the electron beam axis increases with increasing distances of the electron beam from photocathode 2. To provide an electrode 20 with an accelerating field, it is d.c. biased positively relative to photocathode 2 while a decelerating field is provided to electrode 28 by biasing it with a positive d.c. voltage that is considerably less than the d.c. voltage between electrode 20 and the photocathode 2. The photocathode is conveniently maintained at ground potential V,,, while the electrode 20 voltage is referred to as V A resistive voltage divider chain 31, derives positive voltage V less than V,, which is applied to decelerating focus electrode 28. Other intermediate voltages V, and V are also available from the voltage divider chain for reasons that will become apparent.

Between the electron emitting surface 2 and the first focus electrode 20 are disposed two additional electrodes l9 and 21. Electrode 19 is a cathode accelerator ring which aids in establishing high voltage gradients at the electron emitting surface 2. Electrode 19 is maintained at positive accelerating potential V supplied from voltage divider chain 31; potential V is considerably less than the voltages of electrodes 20 and 28. Frusto-conical field shaping electrode ring 21, disposed downstream from the cathode accelerating ring, is maintained at positive potential V supplied from the voltage divider chain; potential V,, is higher than the potential of the cathode accelerator but is still considerably less than the potential at the first focus electrode. The field shaping electrode 21 aids in converting fiat equipotential lines 24 in the vicinity of electron emitting surface 2 to spherical lines 25. The accelerating and decelerating focus electrodes 20 and 28, being of greater potential than that of the electrode 21, further accelerate the electrons toward target 36 while initially providing beam convergence.

The electric field lines are normal to the equipotential lines and the electrons emitted from the electron emitting surface 2 tend to follow the electric field lines passing through the equipotential lines. While the electron beam passes through the equipotential lines 24, trajectories 16 and 18 are initially straight to minimize chromatic aberration and geometric distortion. The beam begins to converge in the vicinity of lines 25 and is focused to almost complete convergence by the relatively high electric field within the aperture 22 of the first focus electrode 20. The region 26 between the electron emitting surface 2 and the first focus electrode 20, therefore, acts as a lens which would introduce some geometric distortion and chromatic aberration if the electron beam were focused in proximity to aperture 22. Because of decelerating electrode 28, the beam is not focused to convergence at aperture 22 and focus electrode 20 may be operated at a substantially higher voltage than possible in prior art systems; in fact, the voltage of electrode 20 may be up to double that usable if it were the only focus electrode, in order to reduce chromatic aberrations.

To provide a strong decelerating field, electrode 28 has an aperture 30 which is, in general, somewhat smaller than aperture 22, whereby the electron beam has a greater cross section as it passes through the electrode 20 than when it passes through electrode 28. P0- tential V which is applied to electrode 28 from the voltage divider chain 31, is somewhat less than V but considerably greater than the voltage V,, on the field shaping electrode 21. Since the potential applied to electrode 28 is less than the potential on electrode 20, the electrode 28 acts as a decelerator, to, in effect, compensate for the relatively high potential that is applied to electrode 20. Electron trajectories 16 and 18 further converge, although the decelerating field causes a reduction in the convergence rates of the trajectories. The second focus electrode is positioned so that at region 32, which is in the neighborhood of aperture 30, the electron beam comes to almost complete convergence. Region 32 may be slightly in front of, slightly in back of, or in a plane including the aperture 30 of electrode 28. Since chromatic aberration causes a band of trajectories, region 32 is not actually a point, but is a small circle of least confusion.

The region 34 between the first and second focus electrodes is, in effect, a second lens, as is apparent from an examination of equipotential lines 27 in that region. However, this second lens is a diverging, rather than a converging lens because it occurs in a decelerating region. Thus, the second lens corrects for the distortions introduced by the lens in the region 26 between the electron emitting surface 22 and the first focus electrode 20.

Downstream from focus electrode 28 is the electron receiving surface or target 36 on which electron image 38 is formed. This surface might be a phosphor coated surface, in the case of an image intensifier tube, or a target to be scanned by an electron beam, in the case of a camera tube. In fact, it may be any surface adapted to receive an electron image and provide an output in response to that image. The electron receiving surface 36 is maintained at potential V by additional power supply 39 connected between the decelerating electrode 28 and the electron target 36. This additional power supply d.c. biases the electron target 36 to a potential greater than or equal to the potential of the decelerating electrode 28, whereby the region 40 between the second focus electron 28 and the electron receiving surface 36 respectively corresponds to either an accelerating space or a drift space. By adjusting power supply 39, the potential V of electron receiving surface 36 may be varied. This variation of potential causes zooming of the already compensated electron trajectories emanating from the region 32 without disturbing their compensation. As the potential V is increased, trajectories 16 and 18 are bent toward the electron beam axis whereby image 38 is smaller in size.

As an example of the application of the inventive concept to a unity magnification image intensifier, the following data are representative of one embodiment. The useful area of each of the input and output surfaces 2 and 36, respectively, is one inch in diameter and the overall length 42 is 4 inches. With the photocathode 2 at ground potential, accelerating electrode 20 is maintained at 20 kilovolts. The cathode accelerator electrode is maintained at nominally 500 volts, while the field shaping ring 21 is maintained at 1600 volts. The planes of aperture 22 and are approximately threefourths inch apart.

With reference to F G. 2, there is shown a minifying image intensifier wherein a large area spherical electron emitting photocathode surface 2 is used to minimize geometric distortions. A fiber optic plate 44, bonded to the convex side of photocathode 2, converts a planar optical image at optical input surface 46 into a spherical optical image at the convex surface of the photocathode which is then transduced by the photocathode to an electron image. In a minifying system, chromatic aberration is a severe problem since the overall length 42 from input surface 46 to target 36 is appreciably longer than in the case of a unity magnifying tube. Thus, each of the trajectories 16 and 18 has associated with it a fairly wide band of statistical trajectories. The first focusing electrode 20 is located relatively far downstream of photocathode 2 so that a focus region 32 may be obtained behind the electrode 20 and in front of electrode 28 to provide a reduced image on the electron receiving surface 36.

v The first focus electrode 20 is maintained at a relatively high accelerating potential while the decelerating electrode 28 is maintained at a lower potential than electrode 20 thereby causing deceleration in the region 34. As in the case of the unity magnification embodiment to FIG. 1, cathode accelerating ring 19 and field shaping ring 21 are disposed between the electron emitting surface 2 and the first focus electrode 20 and respectively maintained at intermediate accelerating potentials V and V,, that are less than the potentials applied to electrodes 20 and 28 and progressively greater than the voltage of source 2. Rings 19 and 21 convert the gradients between the set of equipotential, generally spherical, surfaces 29 in the vicinity of the photocathode 2 into more elongated equipotential surfaces 31 in the vicinity of electrode 20. The electron receiving surface 36, is generally maintained at a higher potential than the accelerating electrode 28, providing-an acceleration in the region 40. The aperture 30 in the decelerating focus electrode 28 is somewhat smaller than the aperture in the accelerating electrode 20, as is the case in P16. 1.

The minification ratio may be varied by changing the potential of image receiving surface 36 without disturbing the distortion compensation performed by the lens in the decelerating region 34 between the focusing electrodes.

An example of the dimensions and the potentials applied to minifying image intensifier embodiment of the invention is as follows: For a minifying ratio of 9 to l, the input surface 46 is 9 inches in diameter and the output surface 36 is 1 inch in diameter. The overall length 42 of the tube is of the order of 1 foot. With the voltage of the photocathode equal to zero, the first focus electrode 20 is nominally operated at 8 kilovolts, the second focus electrode is nominally operated at 3 kilovolts and the electron receiving surface is nominally operated at 25 kilovolts. The spacing between the first and second focus electrodes is nominally 1 inch. The cathode accelerator electrode 19 is preferably operated at 400 volts while the field shaping ring 21 would be operated at 1,000 volts. By decreasing the voltage on electrode 36 to approximately 19 kilovolts the minification decreases by a factor of approximately one-half to 4.5:1. The decrease in potential difference between electrodes 28 and 36 results in less bending of the electron beam in the region between the decelerating and target electrodes.

While there have been described and illustrated several specific embodiments of the invention, it will be clear that variations in the details of the embodiments specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims. For example, additional field shaping electrodes, not generally termed focus electrodes, might be provided.

What is claimed is:

1. An electron optical system comprising: image input means for providing a source of charged particles corresponding to an image, said means having a charged particle output surface maintained at a first tial, whereby the particles are accelerated from the particle output surface toward the first electrode and through the aperture thereof;

a second electrode having an aperture and located downstream from said first electrode, said second electrode being maintained at a third voltage less than said second voltage and being so shaped, dimensioned and positioned as to cause convergence of said charged particles and focussing thereof in a region adjacent said aperture in said second electrode; and

image output means having a surface downstream from said second electrode on which the particles form an image.

2. The system of claim 1 where the aperture of the second electrode is smaller thatn that of the first electrode.

3. The system of claim 2 where said first and second electrodes are surfaces of revolution which are sloped inwardly and upstream with the electrode apertures centrally located.

4. The system of claim 3 wherein said second electrode is located substantially close to a position where said particles are focused to a circle of least confusion.

5. The system of claim 4 wherein a means for zooming the image of the output means is located downstream of said second electrode.

6. The system of claim 4 wherein said first and second electrodes are frusto-conically shaped.

7. The system of claim 6 in combination with a first ring disposed between the input means and the first electrode and maintained at a fourth potential and a second ring disposed between the first ring and the first electrode and maintained at a fifth potential, the fourth potential being greater than said first potential and said fifth potential being greater than said fourth potential but less than said second potential.

8. A charged particle focusing system for a stream of charged particles comprising:

first electrode means for accelerating and focusing said stream; and

second electrode means located downstream from said focus electrode for decelerating said stream while converging and focusing the stream to a circle of least confusion, substantially near its location.

9. The system of claim 8 wherein said first and second electrode means each have a single aperture through which the stream passes, the aperture of the first electrode means being larger than the aperture of the second.

10. The method of forming an electron image on a surface in response to an electron image emitted from a surface comprising a source of electrons comprising:

accelerating electrons from said surface;

further accelerating and converging said electrons accelerated from the source;

decelerating said further accelerated electrons while converging them substantially to a region of least confusion; and

applying a potential to the target surface of sufficient strength to cause the electron image to form thereon.

11. The method of claim 10 with the additional step of varying the relative potential of the target surface to vary the size of the electron image on the target sur face. 

1. An electron optical system comprising: image input means for providing a source of charged particles corresponding to an image, said means having a charged particle output surface maintained at a first potential; a first electrode having an aperture and maintained at a second potential greater than said first potential, whereby the particles are accelerated from the particle output surface toward the first electrode and through the aperture thereof; a second electrode having an aperture and located downstream from said first electrode, said second electrode being maintained at a third voltage less than said second voltage and being so shaped, dimensioned and positioned as to cause convergence of said charged particles and focussing thereof in a region adjacent said aperture in said second electrode; and image output means having a surface downstream from said second electrode on which the particles form an image.
 2. The system of claim 1 where the aperture of the second electrode is smaller thatn that of the first electrode.
 3. The system of claim 2 where said first and second electrodes are surfaces of revolution which are sloped inwardly and upstream with the electrode apertures centrally located.
 4. The system of claim 3 wherein said second electrode is located substantially close to a position where said particles are focused to a circle of least confusion.
 5. The system of claim 4 wherein a means for zooming the image of the output means is located downstream of said second electrode.
 6. The system of claim 4 wherein said first and second electrodes are frusto-conically shaped.
 7. The system of claim 6 in combination with a first ring disposed between the input means and the first electrode and maintained at a fourth pOtential and a second ring disposed between the first ring and the first electrode and maintained at a fifth potential, the fourth potential being greater than said first potential and said fifth potential being greater than said fourth potential but less than said second potential.
 8. A charged particle focusing system for a stream of charged particles comprising: first electrode means for accelerating and focusing said stream; and second electrode means located downstream from said focus electrode for decelerating said stream while converging and focusing the stream to a circle of least confusion, substantially near its location.
 9. The system of claim 8 wherein said first and second electrode means each have a single aperture through which the stream passes, the aperture of the first electrode means being larger than the aperture of the second.
 10. The method of forming an electron image on a surface in response to an electron image emitted from a surface comprising a source of electrons comprising: accelerating electrons from said surface; further accelerating and converging said electrons accelerated from the source; decelerating said further accelerated electrons while converging them substantially to a region of least confusion; and applying a potential to the target surface of sufficient strength to cause the electron image to form thereon.
 11. The method of claim 10 with the additional step of varying the relative potential of the target surface to vary the size of the electron image on the target surface. 